Embodiments of the invention relate generally to a switching device for switching on/off a current in a current path, and more particularly to micro-electromechanical system based switching devices.
A contactor is an electrical device designed to switch an electrical load ON and OFF on command. Traditionally, electromechanical contactors are employed in control gear, where the electromechanical contactors are capable of handling switching currents up to their interrupting capacity. Electromechanical contactors may also find application in power systems for switching currents and in industrial automation systems as motor starters. However, motors controlled by the contactors may fall under a variety of circumstances and fault currents in power systems are typically greater than the interrupting capacity of the electromechanical contactors, Accordingly, to employ electromechanical contactors in power system applications, it may be desirable to protect the contactor from damage by backing it up with a series device that is sufficiently fast acting to interrupt fault currents prior to the contactor opening at all values of current above the interrupting capacity of the contactor. Further, it may be desirable to detect issues with a motor before a costly breakdown occurs.
With regards to early warning motor protection, previously conceived solutions involved costly monitoring of motors by skilled technicians and/or providing complex motor monitoring equipment alongside already bulky motor starters. For example, conventional motor starters include a set of contacts to control current flow. Some motor starters may further include thermal overload protection and/or a local disconnect. However, conventional motor starters lack control circuitry or logical controls integrated thereon. Therefore, monitoring by skilled technicians may include costly downtime of motor starter equipment and undesirable safety issues. For example, in order to monitor a conventional motor starter, a skilled technician may have to physically inspect the device. Therefore, in order to monitor the motor controlled by the conventional motor starter, a skilled technician may have to physically monitor the motor connections on the motor starter and/or motor itself.
If additional monitoring circuitry is used, such additional circuitry would be in addition to a stand-alone motor starter, it is appreciated that givers the often limited space of conventional control panels or motor control cabinets, additional monitoring equipment would hinder visual inspection and decrease space for future integration needs. Therefore, conventional approaches to early warning motor protection have many drawbacks.
With regards to interrupting capacity issues, previously conceived solutions to facilitate use of contactors in power systems include vacuum contactors, vacuum interrupters and air break contactors, for example. Unfortunately, contactors such as vacuum contactors do not lend themselves to easy visual inspection as the contactor tips are encapsulated in a sealed, evacuated enclosure. Further, while the vacuum contactors are well suited for handling the switching of large motors, transformers and capacitors, they are known to cause undesirable transient overvoltages, particularly when the load is switched off.
Furthermore, the electromechanical contactors generally use mechanical switches. However, as these mechanical switches tend to switch at a relatively slow speed, predictive techniques are employed in order to estimate occurrence of a zero crossing, often tens of milliseconds before the switching event is to occur, in order to facilitate opening/closing at the zero crossing for reduced arcing. Such zero crossing prediction is prone to error as many transients may occur in this prediction time interval.
As an alternative to slow mechanical and electromechanical switches, last solid-state switches have been employed in high speed switching applications. As will be appreciated, these solid-state switches switch between a conducting state and a non-conducting state through controlled application of a voltage or bias, for example, by reverse biasing a solid-state switch, the switch may be transitioned into a non-conducting state. However, since solid-state switches do not create a physical gap between contacts when they are switched into a non-conducing state, they experience leakage current. Furthermore, due to internal resistances, when solid-state switches operate in a conducting state, they experience a voltage drop. Both the voltage drop and leakage current contribute to the generation of excess neat under normal operating circumstances, which may affect switch performance and life. Moreover, due at least in part to the inherent leakage current associated with solid-state switches, their use in circuit breaker applications is not practical.
Accordingly, there exists a need in the art for a current switching circuit protection arrangement and reliable early warning motor protection to overcome these drawbacks.